Carburetor cold enrichment control

ABSTRACT

A carburetor has a conventional bimetallic coiled spring that for cold engine starts and running urges a fast idle cam to a cold position to increase the idle speed to sustain engine operation. A separate lever connected to the choke valve is pulled by engine vacuum to move the choke valve wide open, to lean the air/fuel mixture and reduce emissions. For starting purposes, the separate lever is returned to the cold start position dictated by the fast idle cam.

[1111 3,886,241 [4 1 May 27, 1975 United States Patent 1 Sehubeck 3,800,762 4/1974 McCullough.................. 261/39 B X 3,831,567

[ 4] CARBURETOR COLD ENRICHMENT 8/1974 Freismuth et al. 261/39 B X CONTROL [75] Inventor:

Phillip A. Schubeck, Woodhaven, Mich.

[73] Assignee:

Ford Motor Company, Dearborn, Mich.

L. Zerschling ABSTRACT 221 Filed: Mar. 6, 1974 211 Appl. No.: 448,795

A carburetor has a conventional bimetallic coiled spring that for cold engine starts and running urges a fast idle cam to a cold position to increase the idle speed to sustain engine operation. A separate lever connected to the choke valve is pulled by engine vacuum to move the choke valve wide open, to lean the air/fuel mixture and reduce emissions. For starting 5 References Cited purposes, the separate lever is returned to the cold UNITED STATES PATENTS start position dictated by the fast idle cam. 3,529,585 9/1970 Stoltman......................

26'1/52 X 13 Claims, 6 Drawing Figures PATENTEU MAY 2 7 I975 SHEET FICILL PATENTEDM) 27 ms SHEET FIG.2

W a v MAMA @m a PATENTEBHAYZ? I975 I. I 3 5;241-

FIG.5

CARBURETOR COLD ENRICHMENT CONTROL This invention relates in general to a motor vehicle type carburetor. More particularly, it relates to a construction for controlling the opening of the choke valve independently of the positioning of the throttle valve by the fast idle cam during cold engine operation.

Most conventional carburetors have an automatic choke system for enriching the carburetor air/fuel mixture during cold engine operation to maintain good engine driveability. In most instances, a fast idle cam operably rotated by a thermostatically responsive coiled spring is positioned in the path of closing movement of the throttle valve. During cold engine operation, the throttle valve will be opened wider than the normal engine idle speed position to allow sufficient fuel/air mix ture into the engine to sustain engine operation. Simultaneously, the coiled spring moves the choke valve towards a closed position for engine starting, and immediately after the engine has reached a sustained operation, a servo cracks open the choke valve to a position leaning the air/fuel mixture.

The basic automatic choke construction described above is sometimes supplemented by an additional servo mechanism. The latter, after the initial cracking open of the choke valve, pulls open the choke valve to an essentially inoperative position, to reduce emissions. The choke valve, however, in the above instances generally is interconnected with the fast idle cam mechanism. Therefore pulling open the choke valve also causes the fast idle cam to be rotated to an inoperative position. This then permits closure of the throttle plates to their normal engine idle speed position. If, however, the engine operating temperature is still below normal, closure of the throttle plates at this time may decrease the fuel/air mixture flow to a level causing engine stalling. Naturally this is undesirable.

This invention partially separates the fast idle cam mechanism from the choke pulloff servo means. The choke valve can be opened wide to an inoperative position shortly after engine starting, during cold engine operations, to reduce emissions by leaning out the mixture. The throttle valve. however, can be maintained in a fast idle position to permit sufficient flow of air/fuel mixture to sustain engine operation. For restarting, the choke valve can be closed down to a position determined by the fast idle cam.

It is an object of the invention, therefore, to construct a carburetor with a cold enrichment control that provides a limited interconnection between the usual carburetor fast idle cam mechanism and the choke valve pulloff mechanism to permit some independent operation of each with respect to the other, to reduce emissions while maintaining engine driveability.

It is another object of the invention to provide a carburetor cold enrichment control of the type described above that permits opening of the choke valve to an essentially inoperative position soon after the engine has reached a sustained operating condition, to lean the mixture to decrease emissions, without disturbing the fast idle position of the throttle valve as determined by the position of the fast idle cam controlled by a thermostatically responsive coiled spring, to provide increased air/fuel mixture flow during cold engine operation.

Other objects, features and advantages of the invention will become more apparent upon reference to the succeeding, detailed description thereof, and to the drawings illustrating the preferred embodiment thereof, wherein:

FIG. l is a cross-sectional view of a portion of a carburetor embodying the invention;

FIG. 2 is a perspective elevational view of the carburetor shown in FIG. 1; and

FIG. 3 is a side elevational view of a portion of the FIG. 2 showing.

FIG. I is obtained by passing a plane through approximately one-half of a known type of two-barrel. downdraft type carburetor. The portion of the carburetor shown includes an upper air horn section 12, an intermediate main body portion 14, and a throttle valve flange section 16. The three carburetor sections are secured together by suitable means, not shown, over an intake manifold indicated partially at 18 leading to the engine combustion chambers.

Main body portion 14 contains the usual air-fuel mixture induction passage 20 having fresh air intakes at the air horn ends, and connected to manifold 18 at the opposite ends. The passages are each formed with a main venturi section22 containing a booster venturi 24 suitably mounted for cooperation therewith, by means not shown.

Air flow through passages 20 is controlled in part by a choke valve 28 unbalance mounted on a shaft 30 rotatably mounted on side portions of the carburetor air horn, as shown. Flow of fuel and air through each passage 20 is controlled by a conventional throttle valve 36 fixed to a shaft 38 rotatably mounted in flange portion 16. The throttle valves are rotated in a known manner by depression of the vehicle accelerator pedal, and move from idle speed positions essentially blocking flow through passages 20 to wide open positions essentially at right angles to the position shown.

Choke valve 28 rotates from a closed position to the nearly vertical, essentially inoperative position shown.

In this latter position, the choke valve provides the minimum resistance to airflow. The rotative position of choke valve 28 is controlled in part by a semiautomatically operating choke mechanism 40. The latter includes a hollow housing portion 42 that is cast as an integral extension of the carburetor throttle flange. The housing is apertured for supporting rotatably one end of a choke valve control shaft 44. The other end of shaft 44 is rotatably supported by a casting 46. A lever 48 is fixed on the left end portion of shaft 44 and has a finger portion 50. The finger portion constitutes a stop and overlies and is in the path of rotative movement of a tang 52. The tang is formed on a choke valve pivot link 54 that is pivotally connected to the end of a rod 56. Rod 56 at its other end is pivoted to a link 58 fixed on choke valve shaft 30. A torsion choke return spring, not shown, is secured to choke shaft 30 under a cover 59. The spring constantly urges the shaft and valve towards a closed position. It will be clear that rotation of shaft 44 in a clockwise direction will engage finger 50 against tang 52 to correspondingly rotate choke valve 28 to open the carburetor air intake, while rotation of shaft 44 and lever 48 in the opposite direction will not affect movement of the choke valve.

An essentially L-shaped thermostatic spring lever 60 has one leg 62 fixedly secured to the right hand end portion of shaft 44. The other lever leg portion 64 is secured to the end 66 of a coiled thermostatic spring element 68 through an arcuate slot, not shown, in an insulating gasket 70. The inner end portion of the coiled spring is fixedly secured on the end of a nipple 74 that is formed as an integral portion of a choke cap 76 of heat insulating material. Nipple '74 is bored as shown to provide hot air passages 78 and 80, passage 78 being connected to an exhaust manifold heat stove, for example. Cap 76 is secured to housing 42 by suitable means, such as the screw 82 shown, and defines an air or fluid chamber 84.

As thus far described, it will be clear that the thermostatic spring element 68 will contract or expand as a function of the changes in ambient temperature conditions of the air entering tube 78, or, if there is no flow. the temperature of the air within chamber 84. Accordingly, changes in ambient temperature will rotate the spring lever 60 to rotate shaft 44 and lever 48 in one or the other directions, as the case may be.

The leg 62 of spring lever 64 is pivotally fixed to the rod 85 of a piston 86. The latter is movably mounted in a bore 87 in housing 42. The under surface of piston 86 is acted upon by vacuum in a passage 88 that is connected to the carburetor main induction passages 20 by a port 89 located just slightly below throttle valve 36. Piston 86, therefore, is always subject to the vacuum existing in the intake manifold passage portion 18.

A cold engine start of a motor vehicle requires a richer mixture than a warmed engine start because considerably less fuel is vaporized. Therefore, the choke valve is shut or nearly shut to increase the pressure drop thereacross and draw in more fuel and less air. Once the engine does start, however, then the choke valve should be opened slightly to lean the mixture to prevent engine flooding as a result of an excess of fuel.

The choke mechanism described above automatically accomplishes the action described. That is, on cold weather starts, the temperature of the air in chamber 84 will be low so that spring element 68 will contract. This will rotate shaft 44 and lever 48 in a counterclockwise direction away from link 54, permitting the choke return spring to move choke valve 28 to a closed or nearly closed position, as desired. Upon cranking the engine, vacuum in passage 88 will not be sufficient to move piston 86 to open the choke valve. Accordingly, the engine will be started with a rich mixture. As soon as the engine is running, high vacuum in passage 88 moves piston 86 downwardly and rotates shaft 44, lever 48 and link 54 a slight amount so that choke valve 28 is slightly opened so that the mixture is then leaned.

FIGS. 2 and 3 show the carburetor as including a supplemental servo device 96 connected to the choke valve link 54. The details of construction as shown in FIGS. 4-6. Servo 96 has a three piece housing consisting of a lefthand check valve and a vacuum line adapter portion 92, a main body portion 94, and a cup shaped shell portion 96.

Portion 92 has a press fit within an annular flange 98 projecting from body portion 94, and contains a vacuum passage 100. The latter is connected by a tube 102 (FIG. 2) to the carburetor bore at a location below the throttle valve to be subject to manifold vacuum at all times.

Passage 100 communicates with an enlarged stepped diameter bore 104 containing a sintered metal annular plug 105. The wall of the housing portion defining bore 104 contains an axially extending keyway like slot 106 (see FIG. communicating around the left end of plug 105 with passage 100. Seating at times against the right end of restrictor 105 is a thin elastomeric check valve 108. It has an outer ring 110 connected by a flexible neck portion 112 (FIG. 5) to an inner flexible disc 114. The disc 114 has a central hole 116 constituting a flow opening. The total restriction to flow through the sintered metal restrictor 106 is chosen to provide the desired delay in communication of vacuum in line 100 to the opposite side of check valve 108, as will become clearer later. The check valve is held against the recess shoulder of housing portion 92 by a retaining ring 118. The latter is held in place as shown by the staked end 120 of the housing.

Body portion 94 contains an annular flange 122 and a central vacuum passage 124. A Belleville type bimetal disc 126 is floatingly mounted within flange 122, and is biased towards the left by a spring 128. The central portion of disc 126 has an elastomeric valve 130 fixed to it that is adapted to seat against the passage 124 to control flow through it. In assembly, an enlarged flexible integral portion 132 of the valve is pulled through a small hole 131 in disc 126 until the valve is seated, as shown.

The bimetal disc 126 is responsive to ambient temperature changes of the servo to flip over-center from the full line position shown, to the dotted line position 133, in a known manner, at say, temperature levels above 75F, for example, to open passage 124. Reduction of the ambient temperature below the predetermined level will again flip-flop the disc to close passage 124.

The disc spring 128 is seated against a cup shaped spring retainer 134. The latter is held in place in the ring flange 122 by a ring 136, and has a central aperture 138.

The housing portions 94 and 96 together define a hollow interior. Between them is clamped the edge of an annular flexible diaphragm that divides the interior into an air chamber 142 and a vacuum chamber 144. Holes 145 in housing portion 96 permit a free communication of chamber 142 with the ambient air pressure surrounding the servo.

A pair of retainers 146 fixedly mount a plunger or link 148 on the diaphragm, a spring 150 normally biasing the plunger to the position shown. The plunger has a lost motion connection to choke lever 48 consisting of an elongated slot 152 that receives the bent end 154 of a link 156 pivotally connected to lever 54.

Completing the construction, a pair of bosses 158 are welded to housing portion 96 for the attachment of a mounting brake 160 that is adapted to be secured at its opposite end to a portion of the throttle valve body portion of the carburetor.

In operation, below 75F ambient, the bimetal disc 126 remains in a valve closed position as shown, and no vacuum is applied from passage 100 into servo chamber 144. Therefore, the servo 90 is inoperative. The choke valve is initially closed by the torsion spring, and cracked open upon engine startup by the pulldown servo 86. Increases in choke housing temperature, caused by the increases in exhaust manifold heat, cause the bimetal 68 to slowly unwind and move lever 48 against tang 52 to move open the choke valve. This is permitted by the sliding movement of the pin 154 in slot 152. The entire choking action will then be controlled solely by the coil 68.

Assume now that the ambient temperature is above 75F. Initially, the same operation occurs as above described. Check valve 108 will still be seated because of the pressure differential acting on opposite sides. Therefore, bleed of air can only occur through the hole 116 past the sintered restrictor 106. However, now vacuum can be applied to chamber 144 because the thermostatically responsive disc 126 has now flipped over center to position 133. This unseats valve 130 and opens passage 124. Manifold vacuum in passage 100 now is applied slowly, as determined by the combined restrictions of restrictor 106 and hole 116, to begin drawing diaphragm 140 and plunger 148 to the left. This will continue without any effect on the choke valve until the pin 154 is engaged by the end of slot 152. Continued leftward movement of the plunger until it bottoms, at dotted line position 162, against retainer 134, will then rotate link 54 away from finger portion 50 of lever 48 and positively open the choke valve.

The maximum delay is scheduled in this case to be approximately forty seconds. It will vary, of course, from the maximum to a minimum as a function of the initial position of the pin 154 in the slot 152, upon engine startup, which will depend upon the initial ambient temperature conditions affecting the force of coil 68.

Assume now that the engine is shut off, or quickly accelerated, so that the manifold vacuum decays suddenly. It is desirable at this time that the servo 90 recycle itself so as to be in a position to provide the desired choking action when restarted or the acceleration phase terminated. The check valve 108 provides this action since the sudden change in pressure conditions new places the higher pressure level in passage 100, and a lower vacuum in chamber 144. This bends or deforms the check valve outwardly to the right off its seat to permit free communication between passage 100 and chamber 144 by flow around the sintered restrictor 106 through the slot 105. This immediately permits spring 150 to stroke the diaphragm and plunger 148 to the right to the positions shown.

In summary, therefore, below an ambient temperature level of 75F, the choke hot air system provides the only control for choking functions. The bimetal coil 68 will unwind, therefore, only as a function of the increased heating by the hot air from passage 78.

Above 75F, however, the conventional exhaust manifold stove heat system constitutes the primary choke control, while the vacuum operated servo device 90 acts as the supplemental force to pull open the choke valve faster than were it being controlled by the primary control alone. This leans the fuel/air mixture earlier than with conventional choke arrangements, and, therefore, lowers undesirable emissions outputs.

Turn now to the means for controlling the fast idle speed position of the throttle valve. As stated previously, during cold engine operation, it is necessary to open the throttle valve wider to allow sufficient extra air/fuel mixture to prevent the engine from stalling due to the extra friction, greater viscosity of the lubricant, etc. As best seen in FIGS. 1 and 2, rotatably mounted on shaft 44 is a conventional fast idle cam 160. The cam has a projection 162 on one side in which is adjustably mounted a screw 164. The screw engages the finger portion 50 of lever 48. The projection 162 also contains a recess, now shown, in which is pressed a weight or ball of predetermined mass. The mass and its location is chosen such that the cam will always fall by gravity in a clockwise direction to follow the movement of lever 48. This will rotate the cam clockwise progressively as the temperature of the bimetal 68 increases.

The opposite side of cam 160 is formed with an edge 168 having three steps 170, 172 and 174. Each contiguous step in counterclockwise circumferential succession has a face that is of less radial extent than the previous one, the last step 174 being followed by an opening 176. The steps and opening; constitute abutments or stops in the path of movement of a screw 178. The screw is adjustably mounted on a lever 180 fixed to throttle shaft 38. The radial depth of opening 176 is chosen such that when the fast idle cam is rotated to engage the screw 178 in the opening, the throttle valve shaft will have rotated the throttle valve to its normal engine idle speed position essentially closing the throttle valve. Engagement of the screw with each of the steps 170, 172 and 174, as the cam rotates upon temperature decreases, then will progressively locate the idle speed position of the throttle valve at a more open position.

Briefly, in operation, the temperature of bimetal 68 determines the rotative position of lever 48. Gravity causes cam screw 164 to engage finger portion 50 upon temporary movement of screw 178 in a counterclockwise direction by opening the throttle plate. This determines which step 170, 172 or 174, or whether opening 176, will be engaged by screw 178, and, therefore, what the idle speed position of throttle valve 36 will be.

In overall operation, between temperature levels of say F. and F., the bimetallic coiled spring 68 will have rotated lever 48 to some clockwise position similar to that shown in FIG. 2. Subsequent depression of the accelerator pedal will pivot the throttle valve shaft 38 counterclockwise to move screw 178 away from the fast idle cam 160. This will permit the cam to fall by gravity against the finger portion 50 of lever 48. Release of the accelerator pedal then permits the screw to engage whichever step is opposite the screw. This then locates the throttle valve for a fast idle throttle setting predetermining the volume of air/fuel mixture to flow into the engine during cold engine operation at this temperature.

Simultaneously, the clockwise rotation of lever 48 will abut the finger portion 50 with the tang 52 of link 54 to open the choke valve by the correct amount for starting at this temperature. Upon engine starting, engine vacuum is applied to piston 86, which rotates choke shaft 44 a predetermined amount. This rotates lever 48, moving portion 50 against tang 52 to crack open the choke valve by the same degree. The cranking mixture then is leaned to a less rich mixture preventing stalls. Shortly after this operation, vacuum applied through the delayed portion of servo 96 pulls the link 54 clockwise an amount opening the choke valve 28 to an inoperative essentially vertical position. This lessens the emission of undesirable elements by leaning the mixture considerably. This operation is permitted without causing stalls ofthe vehicle because of the freedom of link 54 to pivot clockwise away from the finger portion 50 of lever 48. Thus, even though the choke valve is fully open, the throttle valve is still maintained in a fast idle speed position, to maintain sufficient flow to sustain engine operation during cold engine conditions.

While the invention has been shown and described in its preferred embodiment, it will be clear to those skilled in the arts to which it pertains that many changes and modifications may be made thereto without departing from the scope of the invention.

1 claim:

1. A cold engine enrichment control for use in a carburetor having an air/fuel induction passage open at one end to clean air at essentially atmospheric pressure and connected to an engine intake manifold at the opposite other end to be subject to the changes in engine vacuum, a throttle valve mounted for movement across the other end between closed and fully open positions to control the flow of fuel and air through the passage, and a choke valve anterior of the throttle valve mounted for movement across the one end between closed and open inoperative positions to control the flow of air into the passage, the control including first means operably connected to the choke valve and operable above a predetermined temperature upon the engine attaining a running condition to move the choke valve to an open inoperative position to increase airflow and decrease emissions, and second temperature responsive means operably positioned in the path of closing movement of the throttle valve and variably movable as a function of temperature changes independently of the first means to restrict the closing movement of the throttle valve to maintain the throttle valve open to a beyond normal engine idle speed position to increase the flow of the air/fuel mixture, to prevent engine stallings.

2. A control as in claim 1, the first means including an engine vacuum servo operable to pull open the choke valve.

3. A control as in claim 1, the second means including a throttle shaft, a lever secured to the throttle shaft, a fast idle cam rotatably mounted on the shaft and having an abutment surface with varying radial extent steps engagable individually by the lever, and a temperature responsive member effecting rotation of the cam as a function of temperature changes to determine the open position of the throttle valve.

4. A control as in claim 3, the first means comprising an engine vacuum controlled servo operable to pull open the choke valve.

5. A control as in claim 4, the first means also including a second engine vacuum actuated servo to move the choke valve to an initial slightly open position less than the inoperative position prior to the first servo moving the choke valve.

6. A control as in claim 3, including a bimetallic temperature responsive coiled spring having an end rotatably movable as a function of temperature changes, a member rotated by the spring and engaged by the fast idle cam to restrict the rotative movement of the cam as a function of the movement of the spring.

7. A control as in claim 6, including means on the first means interacting with the member to prevent return movement at times of the first means to a closed choke position.

8. A control as in claim 7, the first means comprising an engine vacuum actuated servo operable to pull open the choke valve.

9. A control as in claim 8, the first means also including a second engine vacuum actuated servo to move the choke valve to an initial slightly open position less than the inoperative position prior to the first servo moving the choke valve.

10. A control as in claim 9, including an engine heat source operably connected to the coiled spring to warm the spring as a function of engine temperature changes.

11. A cold enrichment control for a carburetor having an air/fuel induction passage open at an air end to air at essentially atmospheric pressure and connected at its opposite engine end to an engine intake manifold to be subject to the changing vacuum levels therein, the air end having a choke valve rotatably mounted for movement across the air end between a closed position and an open position wherein the choke valve is inoperative to restrict the flow of air into the passage, and a throttle valve secured to a shaft rotatably mounted posterior of the choke valve for movement of the throttle valve across the passage between a normal essentially closed engine idle speed position and beyond towards a wide open throttle position to control the volume of flow of an air/fuel mixture through the passage as a function of the position of the throttle valve, the control comprising means operable during cold engine operations to position the choke valve independently of the position of the throttle valve, to improve emissions while minimizing engine stalling, the last mentioned means including,

abutment means secured to the throttle shaft, a second shaft, a fast idle cam rotatably mounted on the second shaft and having an edge formed with steps progressively greater in radial extent than one which when engaged by the abutment means would permit closure of the throttle valveto the normal engine idle speed position, whereby the throttle valve will be progressively opened beyond the latter position as a function of the particular step engaged by the abutment means, spring means biasing the abutment means and throttle valve towards a closed position and into individual engagement with one of the steps, a thermostatically responsive coiled spring operatively rotatingthe fast idle cam to positions operatively moving the throttle valve to positions progressively more open than the normal idle speed position as a function of decreases in temperature from a predetermined level, first means operatively connected to the choke valve, engine vacuum actuated servo means connected to the first means to open the choke valve to the inoperative position upon the engine attaining a normal running condition, and means providing a limited unidirectional interconnection between the fast idle cam and choke valve permitting opening of the choke valve and operation of the fast idle cam independent of each other.

12. A control as in claim 11, the last mentioned means including stop means operatively connected to the fast idle cam and engagable with the first means to control the return movement of the choke valve towards a closed position as a function of temperature changes upon engine shutdown.

13. A control as in claim 12, including second vacuum actuated servo means operatively connected to the fast idle cam to effect engagement of the first means to crack open the choke valve beyond a closed position in response to engine start-up. 

1. A cold engine enrichment control for use in a carburetor having an air/fuel induction passage open at one end to clean air at essentially atmospheric pressure and connected to an engine intake manifold at the opposite other end to be subject to the changes in engine vacuum, a throttle valve mounted for movement across the other end between closed and fully open positions to control the flow of fuel and air through the passage, and a choke valve anterior of the throttle valve mounted for movement across the one end between closed and open inoperative positions to control the flow of air into the passage, the control including first means operably connected to the choke valve and operable above a predetermined temperature upon the engine attaining a running condition to move the choke valve to an open inoperative position to increase airflow and decrease emissions, and second temperature responsive means operably positioned in the path of closing movement of the throttle valve and variably movable as a function of temperature changes independently of the first means to restrict the closing movement of the throttle valve to maintain the throttle valve open to a beyond normal engine idle speed position to increase the flow of the air/fuel mixture, to prevent engine stallings.
 2. A control as in claim 1, the first means including an engine vacuum servo operable to pull open the choke valve.
 3. A control as in claim 1, the second means including a throttle shaft, a lever secured to the throttle shaft, a fast idle cam rotatably mounted on the shaft and having an abutment surface with varying radial extent steps engagable individually by the lever, and a temperature responsive member effecting rotation of the cam as a function of temperature changes to determine the open position of the throttle valve.
 4. A control as in claim 3, the first means comprising an engine vacuum controlled servo operable to pull open the choke valve.
 5. A control as in claim 4, the first means also including a second engine vacuum actuated servo to move the choke valve to an initial slightly open position less than the inoperative position prior to the first servo moving the choke valve.
 6. A control as in claim 3, including a bimetallic temperature responsive coiled spring having an end rotatably movable as a function of temperature changes, a member rotated by the spring and engaged by the fast idle cam to restrict the rotative movement of the cam as a function of the movement of the spring.
 7. A control as in claim 6, including means on the first means interacting with the member to prevent return movement at times of the first means to a closed choke position.
 8. A control as in claim 7, the first means comprising an engine vacuum actuated servo operable to pull open the choke valve.
 9. A control as in claim 8, the first means also including a second engine vacuum actuated servo to move the choke valve to an initial slightly open position less than the inoperative position prior to the first servo moving the choke valve.
 10. A control as in claim 9, including an engine heat source operably connected to the coiled spring to warm the spring as a function of engine temperature changes.
 11. A cold enrichment control for a carburetor having an air/fuel induction passage open at an air end to air at essentially atmospheric pressure and connected at its opposite engine end to an engine intake manifold to be subject to the changing vacuum levels therein, the air end having a choke valve rotatably mounted for movement across the air end between a closed position and an open position wherein the choke valve is inoperative to restrict the flow of air into the passage, and a throttle valve secured to a shaft rotatably mounted posterior of the choke valve for movement of the throttle valve across the passage between a normal essentially closed engine idle speed position and beyond towards a wide open throttle position to control the volume of flow of an air/fuel mixture through the passage as a function of the position of the throttle valve, the control comprising means operable during cold engine operations to position the choke valve independently of the position of the throttle valve, to improve emissions while minimizing engine stalling, the last mentioned means including, abutment means secured to the throttle shaft, a second shaft, a fast idle cam rotatably mounted on the second shaft and having an edge formed with steps progressively greater in radial extent than one which when engaged by the abutment means would permit closure of the throttle valve to the normal engine idle speed position, whereby the throttle valve will be progressively opened beyond the latter position as a function of the particular step engaged by the abutment means, spring means biasing the abutment means and throttle valve towards a closed position and into individual engagement with one of the steps, a thermostatically responsive coiled spring operatively rotating the fast idle cam to positions operatively moving the throttle valve to positions progressively more open than the normal idle speed position as a function of decreases in temperature from a predetermined level, first means operatively connected to the choke valve, engine vacuum actuated servo means connected to the first means to open the choke valve to the inoperative position upon the engine attaining a normal running condition, and means providing a limited unidirectional interconnection between the fast idle cam and choke valve permitting opening of the choke valve and operation of the fast idle cam independent of each other.
 12. A control as in claim 11, the last mentioned means including stop means operatively connected to the fast idle cam and engagable with the first means to control the return movement of the choke valve towards a closed position as a function of temperature changes upon engine shutdown.
 13. A control as in claim 12, including second vacuum actuated servo means operatively connected to the fast idle cam to effect engagement of the first means to crack open the choke valve beyond a closed position in response to engine start-up. 